1. Field of the Invention
This invention relates to an optical fiber-fixing structure of a ferrule, and more particularly, to an improved optical fiber-fixing structure of a ferrule in which a sheath of an optical fiber is provisionally fixed in a press-fitted condition.
2. Related Art
An optical communication structure has heretofore been used for transmitting a large quantity of information such as voice, images and data pulses. In the optical communication structure, usually, an electrical signal is converted into light, and this light is propagated through an optical fiber and emerges to be again converted into an electrical signal, and by doing so, a large amount of information is transmitted.
In the optical communication structure, various kinds of optical connectors are used for connecting the optical fibers together and for connection to a receptacle having a light-emitting element.
An optical connector includes an optical fiber, subjected to an end processing, a ferrule mounted on an end portion of the optical fiber, and a housing for receiving the optical fiber and ferrule. The optical fiber and the ferrule comprise the following members.
As shown in FIG. 3, the optical fiber 1 comprises a conductor 2 of a circular cross-section consisting of a core and a clad, and primary and secondary sheaths (often referred to as "jacket") 3 and 4 formed in layers on an outer peripheral surface of the conductor 2.
In the drawings, the optical fiber is subjected to an end processing by which predetermined lengths of the sheaths are peeled and removed from the end portion, so that the conductor 2 and the primary sheath 3 are exposed.
A ferrule 5, which is to be mounted on the optical fiber 1 subjected to the above end processing, is made of a synthetic resin, and is formed into a generally stepped, cylindrical shape. The ferrule 5 includes two receiving portions 6 and 7 for respectively receiving the primary sheath 3 and the conductor 2, wherein the receiving portion 6 has a larger inner diameter than the receiving portion 7.
The receiving portions 6 and 7 are coaxial with each other, and extend continuously through the ferrule 5 from one end 5a thereof to the other end 5b thereof. A tapering surface 7a is formed at a portion of the receiving portion 7 that is connected to the receiving portion 6.
The length of the receiving portion 6 in a direction of a longitudinal axis thereof (i.e., along the direction of the center axis of the ferrule 5) is equal to the length of the exposed primary sheath 3, and the length of the receiving portion 7 in a direction of a longitudinal axis thereof is equal to the exposed conductor 2.
An inner diameter D1 of the receiving portion 6 is slightly larger than an outer diameter D2 of the primary sheath 3 so as to provide an insertion clearance.
An annular flange 5c for engagement with the above-mentioned housing (not shown) is formed on a generally central portion of the outer peripheral surface of the ferrule 5, and projects perpendicularly to the central axis of the ferrule, the annular flange 5c being formed on that portion of the ferrule 5 having the receiving portion. A similar flange 5d is also formed on the outer peripheral surface of the ferrule at the one end 5a thereof.
The optical fiber 1 and the ferrule 5, having the above respective constructions, are fixed together through the following steps.
First, an adhesive (not shown) is coated on that portion (indicated by the range H) of the optical fiber 1, extending from the distal end of the conductor 2 to the step portion between the primary and secondary sheaths 3 and 4, over an entire area thereof.
Then, the ferrule 5, being coated with the adhesive, is fitted into the optical fiber 1, through the one end 5a thereof, and is mounted thereon as shown in FIG. 4.
Then, the above adhesive is solidified by air drying or forced drying.
In the above conventional technique, the optical fiber 1 and the ferrule 5 are fixed together by the use of the adhesive, and therefore there is a possibility that the following situations occur before the adhesive is dried.
Firstly, the small insertion clearance (D1-D2, see FIG. 3) is formed between the receiving portion 6 and the primary sheath 3, and therefore before the adhesive is completely dried, the ferrule 5 can be displaced out of position unless this assembly is handled carefully, particularly when being moved. (If the ferrule strikes against something, it may be disengaged from the optical fiber).
This careful handling adversely affects the efficiency of the operation, and therefore, naturally, it is not desirable to continue to use this step.
Even if the handling is performed carefully, it is still necessary to provide the step of checking whether or not the ferrule 5 has been displaced, and therefore the arrangement of personnel responsible for this checking step must be taken into consideration, and this further lowers the efficiency of the operation.
Therefore, there is still room for improvement for the manner and structure in which the optical fiber 1 and the ferrule 5 are fixed together.
A second point to be noted is the strength of fixing between the optical fiber 1 and the ferrule 5. The air included in the adhesive is not fully removed therefrom when the ferrule is mounted on the optical fiber, and when the adhesive is dried in this condition, there is provided, in some cases, a product in which the fixing strength is much lowered.
More specifically, the air, included in the adhesive, is formed into bubbles, and if many bubbles are disposed in contact with the bonding surface, the area of bonding is reduced by an amount corresponding to the number of these bubbles when the adhesive is dried, and as a result the fixing strength is lowered.
To deal with this, it may be proposed to form air vent holes through the peripheral wall of the ferrule 5. However, the bubbles are formed at random on the bonding surface, and therefore this is not a satisfactory countermeasure. In addition, since pins for forming the air vent holes are formed on a mold, the mold would have a complicated shape, and thus, is not practical.
Currently, the ferrule 5 is gradually mounted on the optical fiber while being rotated in directions of arrow P (FIG. 5) so as to remove the air.
However, this method is effective only when the above-mentioned clearance has a certain size, and this clearance (D1-D2) is generally small, and therefore, the air cannot be satisfactorily removed from the adhesive.
Moreover, this method entails a very cumbersome operation, requiring much time, which lowers the efficiency of the operation.